Anatalavis oxfordi, Olson

DYKE, GARETH J., 2001, The Fossil Waterfowl (Aves: Anseriformes) from the Eocene of England, American Museum Novitates 3354, pp. 1-16 : 2-5

ANATALAVIS OXFORDI Olson, 1999

The holotype specimen ( BMNH PAL 5922 ; fig. 1A–C ) of Anatalavis oxfordi Olson is one of the best preserved fossil birds known from the London Clay Formation. Unfortunately, however, it is also one of the most fragile fossil bird specimens known. Although both the partially complete skull, sternum, and pelvis (along with a number of forelimb elements) have been prepared, studying this material is complicated by the delicate preservation of the material. This specimen was collected from bed A of the London Clay at Walton­on­the­Naze, Essex, England (King, 1981) in 1991 (Olson, 1999) .

FOSSIL MATERIAL: BMNH PAL 5922 ( fig. 1A–C ), an incomplete skeleton consisting of skull ( fig. 1A ), incomplete rostral portion of mandible, pterygoid, atlas, axis, thoracic and caudal vertebrae, complete furcula ( fig. 1B ), left and right coracoids ( fig. 1C ), left and right scapulae, incomplete sternum (lacking posterior portions), incomplete anterior por­ tion of pelvis, complete right and incomplete left humeri, distal ends of left radius and ulna, left ulnare, complete left carpometacarpus, complete phalanges, and minor digits of left forearm. For complete description and illustration of this material, see Olson ( 1999).

TAXONOMIC HISTORY: Olson ( 1999) considered the holotype specimen of Anatalavis oxfordi and referred it to within the extant family Anseranatidae , as the sole member of a new subfamily, the Anatalavinae ( fig. 2 ). The only other member of this family of anseriforms is the extant Australian Magpie Goose, Anseranas semipalmata . On the basis of this referral, a number of significant conclusions were proposed by Olson ( 1999): (1) that the fossil record of this extant family can be traced back to the Lower Eocene of the northern hemisphere, and (2) the origin of this group of waterfowl can be inferred to have occurred in the northern hemisphere (of course, the distribution of the family is currently restricted to the southern continents). Olson ( 1999) further proposed that the early fossil record of the order Anseriformes in the northern hemisphere may have been dominated by anseranatid­like taxa during this time, to be replaced later by the members of the Anatidae , or true ducks.

Referral of the holotype specimen of Anatalavis oxfordi to the Anseranatidae was made by Olson (1999: 232) on the basis of the following osteological characters: the Vshaped conformation of the furcula, the size of the symphysis ( fig. 1B ), and the presence of a distinct foramen pneumaticum on the dorsal surface of the distal coracoid ( fig. 1C ). All of these features were considered by Olson ( 1999) to be uniquely derived within the Anseranatidae . In particular, with regard to the presence of a foramen pneumaticum on the dorsal coracoid, he noted that (p. 232) ‘‘although a similar condition exists in modern Anhimidae [screamers], which have one of the most pneumatized skeletons in any group of birds, this foramen is absent in Eocene Anhimidae , which are evidently com­ pletely nonpneumatic’’. Although it is true that the this character is present in the extant screamers (Livezey, 1997), the extent of variation of this feature in the known fossil forms is unclear since little material has been described to date. Indeed, examination of material described and illustrated by Alvarenga ( 1999) as a fossil screamer from the Oligocene of southeastern Brazil shows that this foramen is clearly present (albeit broken distally: Alvarenga, 1999: figs. 3 , 4 ). Although Olson ( 1999) cited as yet unpublished material of fossils referred informally by some to screamers or similar birds from the Eocene of England and Wyoming (p. 232), clearly the distribution (and primitive absence) of this character requires further investigation. In the absence of a cladistic analysis including all available osteological evidence, as well as the characters cited by Olson ( 1999), the referral of this specimen to the extant family Anserantidae cannot be considered unequivocal.

PHYLOGENETIC ANALYSIS: The phylogenetic relationships of the exant genera of waterfowl have been considered most recently by Ericson ( 1997) and Livezey ( 1997, 1998). Both of these workers included the fossil Presbyornis pervetus Wetmore (see above) within osteological character analyses and agreed that it should be placed as the sister taxon to the Recent Anatidae (true ducks: Ericson, 1997; Livezey, 1997).

By use of the osteological matrix present­ ed by Livezey ( 1998), the holotype specimen of Anatalavis oxfordi (BMNH PAL 5922) was coded for inclusion in a parsimony analysis. Of the 96 osteological characters presented by Livezey ( 1998), only 22 (23 %) are informative for Anatalavis because of the incomplete and fragile preservation of the holotype. These character codings are given in appendix 1. Some small changes were made to the character­state definitions provided by Livezey ( 1997) for the analysis presented here: characters 3 (presence / absence of ventral prominence of processus paraoccipitalis relative to plane of os paraspenoidale) and 20 (presence/absence of processus retroarticularis) were amended as outlined to simple binary states because of uncertainties in the coding of these characters in the skull of Anatalavis (these changes do not affect the results of the analysis). Consideration of these features in the holotype skull will represent an area for further work.

Parsimony analysis of the osteological dataset presented by Livezey ( 1997) by use of the software package PAUP (version 4.0b; Swofford, 1999: all characters unordered using Tinamiformes as the outgroup), and including the holotype of Anatalavis oxfordi , resulted in the production of a single most parsimonious tree (MPT) of length 138 steps ( fig. 3 ). The inclusion of Anatalavis within the analysis has no effect on the relationships of the remaining extant taxa and Presbyornis (see Livezey, 1997): the London Clay fossil is hypothesized to occur as the sister taxon to the extant Anatidae and Presbyornis ( fig. 3 ), in a more derived position with respect to the Anseranatidae (Anseranas) . The position for this taxon hypothesized by Olson ( 1999) is not supported by parsimony analysis.

REVISED TAXONOMY OF ANATALAVIS OXFOR­ DI: Although consideration of the phylogenetic position of Anatalavis oxfordi Olson is complicated by incomplete preservation of the holotype, the position of this taxon within the single MPT ( fig. 3 ) is unequivocal. On the basis of the osteological characters (and analysis) presented by Livezey ( 1997), monophyly of the waterfowl (order Anseriformes ) is supported by 11 unambiguous characters (see Livezey, 1997, for details), one of which, the presence of a long and ventrally terminating lamina basiparasphenoidalis (relative to the plane of the parasphenoidale; character 3, Livezey, 1997), is preserved in Anatalavis (appendix 1). Olson (1999: 232) noted that the skull of the holotype has all of the features typical of the order Anseriformes , to which it clearly belongs. These include (according to Olson, 1999) a ‘‘duck­billed’’ upper jaw shape; the configuration of the quadrato­mandibular articulation; an enlarged, deep, curved, retroarticular process; and an enlarged rounded or ovoid ‘‘basipterygoid process’’ on the parasphenoid rostrum, with a correspondingly enlarged facet on the pterygoid.

The position of Anatalavis within the clade Anseres (true waterfowl; see fig. 3 ) can be inferred by the presence of a prominent crista fossa parabasalis of the exoccipitale (derived state for character 2 of Livezey, 1997) and a prominent processus coronoideus of the mandible (derived state for character 18 of Livezey, 1997). A sister group relationship between Anatalavis , Presbyornis , and the clade Anatidae (true ducks) can be hypothesized because of the presence of a spina externa (as a compressed phlange) on the rostral end of the sternum (derived state for character 60 of Livezey, 1997), and an elongate acromion on the cranial end of the scapula (reversal within Anseriformes for character 68 of Livezey, 1997).

Both of the features discussed by Olson ( 1999) in support of the referral of Anatalavis to the Anseranatidae were included within the osteological character matrix of Livezey ( 1997) and have been demonstrated to have a wider distribution within Anseriformes . The presence of a hypocleideum on the furcula (character 67 of Livezey, 1997), although seen in Anatalavis and Anseranas , is hypothesized to have been present primitively within the group: this character, tested by the congruence of others, has been secondarily lost in Anhima , Chauna , Presbyornis , and the Anatidae . The second of the two features discussed by Olson ( 1999), the presence of a foramen pneumaticum on the dorsal surface of the coracoid (character 70 of Livezey, 1997), is hypothesized to diagnose the clade Anseriformes + Galliformes and is lost in Presbyornis and the Anatidae . Hence, both of the osteological features presented by Olson ( 1999) in support of the referral of Anatalavis to the Anseranatidae are primitive among Anseriformes .

Figures

Fig. 1. Parts of the holotype specimen of Anatalavis oxfordi Olson (BMNH PAL 5922). A, skull in lateral views; B, furcula; C, coracoid in dorsal and medial views. Scale bars are 10 mm. HYP indicates hypocleidium (apophysis furculae); FP, foramen pneumaticum.

Fig. 2. Phylogenetic position of Anatalavis oxfordi proposed by Olson (1999).

Fig. 3. Single tree (138 steps) resulting from reanalysis of Livezey (1997) and including the London Clay fossil Anatalavis (consistency index CI = 0.761; retention index RI = 0.815).

Fig. 4. Some of the fossil material referred to Headonornis hantoniensis Harrison and Walker. A, holotype right coracoid in dorsal view (BMNH PAL 30325); B, referred distal end of left humerus in cranial view (BMNH PAL 5105); C, referred right humerus in caudal view (BMNH PAL 3686). Scale bars are 5 mm.